Spin hall effect magnetic apparatus, method and applications

ABSTRACT

An ST-MRAM structure, a method for fabricating the ST-MRAM structure and a method for operating an ST-MRAM device that results from the ST-MRAM structure each utilize a spin Hall effect base layer that contacts a magnetic free layer and effects a magnetic moment switching within the magnetic free layer as a result of a lateral switching current within the spin Hall effect base layer. This resulting ST-MRAM device uses an independent sense current and sense voltage through a magnetoresistive stack that includes a pinned layer, a non-magnetic spacer layer and the magnetic free layer which contacts the spin Hall effect base layer. Desirable non-magnetic conductor materials for the spin Hall effect base layer include certain types of tantalum materials and tungsten materials that have a spin diffusion length no greater than about five times the thickness of the spin Hall effect base layer and a spin Hall angle at least about 0.05.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to, and derives priority from: (1) U.S.Provisional Patent Application Ser. No. 61/524,998, filed Aug. 18, 2011;(2) U.S. Provisional Patent Application Ser. No. 61/534,517, filed Sep.14, 2011; (3) U.S. Provisional Patent Application Ser. No. 61/545,705,filed Oct. 11, 2011; and (4) U.S. Provisional Patent Application Ser.No. 61/619,679, filed Apr. 3, 2012, each titled Spin Hall EffectApparatus, Method and Applications, the content of each of whichprovisional patent application is incorporated herein fully byreference.

STATEMENT OF GOVERNMENT INTEREST

The research that lead to the embodiments as disclosed herein and theinvention as claimed herein was funded by the United States ArmyResearch Office under award W911NF-08-2-0032, the United States DefenseAdvanced Research Project Agency, under award HR0011-11-C-0074, and theUnited States Office of Naval Research under award N00014-10-1-0024. TheUnited States Government has rights in the invention claimed herein.

BACKGROUND

1. Field of the Invention

Embodiments relate generally to magnetic memory devices, such as but notlimited to magnetic random access memory (MRAM) devices. Morespecifically, embodiments relate to magnetic memory devices, such as butnot limited to magnetic random access memory (MRAM) devices, withenhanced performance.

2. Description of the Related Art

Magnetic random access memory (MRAM) devices comprise a class ofnon-volatile, fast and energy-efficient data storage devices that areexpected to be capable of achieving very high levels of magnetic datadensity. Each MRAM cell stores one binary bit of data. A centralcomponent of an MRAM device cell is a thin film magnetoresistiveelement, which is a combination of thin film materials whose electricalresistance depends upon the relative magnetic orientation of two ormore, but generally only two, thin film ferromagnetic material layers.One of the thin film ferromagnetic material layers typically has a fixedmagnetic orientation and is referred to as a pinned layer (PL), whilethe other of the thin film ferromagnetic material layers has aswitchable magnetic orientation and is referred to as a free layer (FL).

A further sub-category of MRAM devices is spin-torque MRAM (ST-MRAM)devices. In comparison with more conventional MRAM devices, ST-MRAMdevices utilize a spin torque generated by an electrical current (i.e.,rather than a magnetic field generated by an electrical current) toswitch the relative magnetic orientation of a free layer (FL) withrespect to a pinned layer (PL).

As integrated circuit device dimensions have decreased and integratedcircuit device density has increased, it has become desirable to providemore efficient and more reliable MRAM structures including ST-MRAMstructures, as well as methods for fabrication thereof and operationthereof for the more efficient and more reliable MRAM structuresincluding the ST-MRAM structures.

SUMMARY

Embodiments provide an ST-MRAM structure, a method for fabricating theST-MRAM structure and a method for operating an ST-MRAM device thatresults from the ST-MRAM structure.

The ST-MRAM structure and related methods in accordance with theembodiments use a base layer having an enhanced spin Hall effect (i.e.,a spin Hall effect base layer (SHE base layer)) which is located andformed contacting a free layer within the ST-MRAM structure. The ST-MRAMstructure and related methods in accordance with the embodiments arepredicated upon use of a lateral switching current applied with respectto the SHE base layer that provides for magnetic alignment switchingwithin the free layer. A sensing current and a sensing voltage may bemeasured or applied perpendicularly through a thin film magnetoresistiveelement stack that sequentially comprises a pinned layer, a non-magneticspacer layer and the free layer that contacts the SHE base layer withinthe ST-MRAM structure.

Thus, the ST-MRAM structure and device in accordance with theembodiments provide for: (1) an in-plane lateral switching currentwithin an SHE base layer that switches a magnetic orientation of a freelayer with respect to a pinned layer; in conjunction with (2) aperpendicular to plane sensing current and sensing voltage through athin film magnetoresistive element stack comprising the pinned layer, anon-magnetic spacer layer and the free layer (that contacts the SHE baselayer), when operating the ST-MRAM structure and device.

To achieve the foregoing results, an SHE base layer within an ST-MRAMstructure and device in accordance with the embodiments comprises anon-magnetic conductor material that has: (1) a spin Hall angle greaterthan about 0.05 (and more preferably greater than about 0.10); and (2) amaximum thickness no greater than about 5 times a spin diffusion length(and more preferably from about 1.5 to about 3 times the spin diffusionlength) within the non-magnetic conductor material. For devicegeometries in which the equilibrium magnetization of the magnetic freelayer is in the sample plane, the non-magnetic conductor material withthe foregoing spin Hall angle should also have the property that: (3a)its presence adjacent to and contacting the magnetic free layerincreases a magnetic damping of the magnetic free layer by no more thana factor of 2 above an intrinsic value for the magnetic free layermaterial. For device geometries in which the equilibrium magnetizationof the magnetic free layer is perpendicular to the sample plane, thenon-magnetic conductor material with the foregoing spin Hall angleshould have the property that: (3b) the interface between it and themagnetic free layer contributes a perpendicular magnetic anisotropy tothe magnetic free layer that allows the anisotropy energy of themagnetic free layer to achieve an optimized value between 40 k_(B)T and300 k_(B)T, where k_(B) is the Boltzmann constant and T is thetemperature.

The embodiments thus comprise a magnetic tunnel junction that allows asensing current to flow perpendicular to the plane of a plurality offilms, and an adjacent non-magnetic metallic strip comprising a materialhaving a comparatively strong spin Hall effect (SHE) that can carrycurrent flowing in the film plane. Metallic elements that have acomparatively large spin Hall angle include but are not limited to Taand W, as is discussed further below. Ta and W in their high-resistivitybeta phases are particularly suited for manipulating in-plane polarizedmagnetic free layers, while Ta and W in their high-resistivity betaphases as well as Pt are suited for manipulating magnetic free layerswith their magnetization oriented perpendicular to the thin film plane.Alloys of these and other elements can be formed that also have strongSHE effects and can be used within the context of the embodiments. Themagnetic tunnel junction is comprised of a ferromagnetic layer withfixed magnetization direction (i.e., the pinned layer (PL)), anotherferromagnetic layer that has a magnetization which is free to rotateunder spin current or magnetic field (i.e., the free layer (FL)), and atunneling barrier or a non-magnetic metal layer (i.e. the non-magneticspacer layer) that separates the free layer and the pinned layer. TheSHE base layer that comprises the non-magnetic strip is in contact withthe free magnetic layer and is located on the opposite side of the freelayer/tunneling barrier interface. The write current flows laterally inthe nonmagnetic strip SHE base layer without any substantial portion ofthe write current passing through the tunnel barrier, while the read orsense current is applied across the magnetic tunnel junction. Themagnetic tunnel junction may also include other layers, for example, thepinned magnetic layer can be pinned with either an antiferromagneticlayer, or by a synthetic antiferromagnetic tri-layer consisting of twothin ferromagnetic layers separated by a thin non-magnetic layer such asRu of a thickness that results in anti-parallel magnetic alignment ofthe two magnetic layers by the indirect exchange interaction, or by someother magnetic pinning method; and/or both the pinned and free magneticlayers may comprise synthetic antiferromagnetic layers or otherferromagnetic multilayers.

For a material with a strong spin Hall effect (SHE), and while not beingbound by any particular theory, when there is a charge current flowinglongitudinally, the so-called spin-orbit coupling between the electronsin the current and the ions in the metal causes electrons that havetheir spin in one orientation to be preferentially deflected into onedirection transverse to the current, while electrons with the oppositespin are deflected in the opposite transverse direction. The net resultis a “spin current” of electrons flowing transverse to the chargecurrent. The spin current direction is determined by the cross productof the spin orientation and the direction of charge current flow. Whenthe spin polarized electrons that form this spin current reach theinterface between the non-magnetic layer SHE base layer where the SHE isgenerated and the FL, these electrons will exert a spin torque on theFL, whereby the magnetization of the FL can be rotated or switched.Since the spin Hall effect occurs in a very thin layer that isimmediately adjacent to (i.e., adjoining) the FL, there is nosignificant loss of spin current by diffusion out the electrical leads.Moreover, the spin Hall effect is superior in the efficiency ofgenerating spin current since it allows torques corresponding to morethan one unit of transferred spin (h/4pi) per electron in the current(where h is Planck's constant and pi is the fundamental constantdetermined by the ratio of the circumference of a circle to itsdiameter). The conventional method of producing spin currents via spinfiltering in a magnetic tunnel junction or magnetic spin valve islimited to torques corresponding to strictly less than one unit oftransferred spin (h/4pi) per electron in the current. As a result, byusing the SHE one can switch a magnet with smaller currents and energycosts compared with the conventional current induced switching, therebyimproving device efficiency. Compared to conventional current inducedswitching in magnetic tunnel junctions, the SHE device geometry alsoprovides a separation of the current paths for reading and writing,which greatly improves the device reliability, presently the mainobstacle to ST-MRAM commercialization.

To read the data stored in a ST-MRAM memory cell in accordance with theembodiments, current is applied perpendicularly across the magnetictunnel junction. Since the magnetization of the FL and PL can be eitherparallel or antiparallel, due to tunneling magnetoresistance the memorycell will be either in the low resistance state or the high resistancestate, where one of these two states can represent the binary data 0 andthe other state the binary data 1.

A particular ST-MRAM structure in accordance with the embodimentsincludes a spin Hall effect base layer located over a substrate. Theparticular structure also includes a magnetic free layer located overthe substrate and contacting the spin Hall effect base layer. Within thestructure, a non-magnetic conductor material that comprises the spinHall effect base layer has: (1) a spin Hall angle greater than about0.05; and (2) a thickness no greater than about 5 times a spin diffusionlength in the non-magnetic conductor material.

Another particular ST-MRAM structure in accordance with the embodimentsincludes a spin Hall effect base layer located over a substrate andincluding two laterally separated terminals. This other particularstructure also includes a magnetoresistive stack located contacting thespin Hall effect base layer and comprising: (1) a magnetic free layerlocated contacting the spin Hall effect base layer; (2) a non-magneticspacer layer located over the magnetic free layer; and (3) a pinnedlayer located over the non-magnetic spacer layer. This other particularstructure also includes a third terminal electrically connected to thepinned layer. Within this other particular structure, a non-magneticconductor material that comprises the spin Hall effect base layer has:(1) a spin Hall angle greater than about 0.05; and (2) a thickness nogreater than about 5 times a spin diffusion length in the non-magneticconductor material.

A particular method for fabricating the ST-MRAM structure in accordancewith the embodiments includes forming over a substrate a spin Halleffect base layer comprising a non-magnetic conductor material having:(1) a spin Hall angle greater than about 0.05; and (2) a thickness nogreater than about 5 times a spin diffusion length in the non-magneticconductor material. The particular method also includes forming over thesubstrate and contacting the spin Hall effect base layer a magnetic freelayer.

A particular method for operating the ST-MRAM structure in accordancewith the embodiments includes providing a magnetic structure comprising:(1) a spin Hall effect base layer located over a substrate and includingtwo laterally separated terminals; (2) a magnetoresistive stack locatedcontacting the spin Hall effect base layer and comprising: (a) amagnetic free layer located contacting the spin Hall effect base layer;(b) a non-magnetic spacer layer located over the magnetic free layer;and (c) a pinned layer located over the non-magnetic spacer layer; and(3) a third terminal electrically connected to the pinned layer. Withinthis method, a non-magnetic conductor material that comprises the spinHall effect base layer has: (1) a spin Hall angle greater than about0.05; and (2) a thickness no greater than about 5 times a spin diffusionlength in the non-magnetic conductor material. This method also includesapplying a switching current to the two laterally separated terminals toswitch a magnetic direction of the free layer with respect to the pinnedlayer.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the embodiments are understoodwithin the context of the Detailed Description of the Embodiments, asset forth below. The Detailed Description of the Embodiments isunderstood within the context of the accompanying drawings, that form amaterial part of this disclosure, wherein:

FIG. 1 shows a schematic perspective-view diagram illustrating a threeterminal ST-MRAM device cell that employs the SHE for a writingoperation in accordance with the embodiments. The ST-MRAM cell iscomprised of a magnetic tunnel junction and a non-magnetic strip with astrong SHE. In this particular embodiment, the non-magnetic strip islocated on the bottom of the device structure.

FIG. 2 shows an alternate embodiment of the embodiment illustrated inFIG. 1, where the non-magnetic strip with the strong SHE is located onthe top of the ST-MRAM device structure.

FIG. 3 shows another alternate embodiment of the embodiment illustratedin FIG. 1, where the equilibrium positions of the magnetic moments ofthe FL and PL are perpendicular to the film plane.

FIG. 4 shows the direction of the injected spin σ, as well as theflowing direction of the charge current J_(c) and spin current J_(s).

FIG. 5 shows the ferromagnetic resonance (FMR) linewidth obtained fromCoFeB (3-4 nm) with Pt (6 nm), Ta (8 nm) or W (6 nm) under differentfrequencies. The damping coefficients α were obtained from linear fit ofthe data shown in FIG. 5.

FIG. 6 shows a schematic diagram of a three terminal spin Hall effectdevice using a tantalum film as an SHE base layer.

FIG. 7 shows experimental data obtained from a three terminal spin Halleffect device with a tantalum SHE base layer, demonstrating SHE inducedmagnetic switching at room temperature of an in-plane-magnetizedferromagnetic layer.

FIG. 8A shows a diagram illustrating the direction of an effective spintorque field B_(ST) under different orientations of a FL magneticmoment. The injected spins are assumed to be along the −x direction asan example.

FIG. 8B shows a diagram illustrating the relationship between thedirection of the effective spin torque field B_(ST) and that of theapplied field B_(ext).

FIG. 9 shows experimental data obtained from a Pt/Co/Al multilayer,demonstrating SHE induced magnetic switching at room temperature of aperpendicularly-magnetized ferromagnetic layer.

FIG. 10 is a variant of the structure shown in FIG. 3, which has anadditional in-plane magnetized ferromagnetic material layer, providingthe in-plane field needed for defining a definite switching direction.

FIG. 11 shows a ratio between injected spin current and charge currentfor Ta and Pt separately. The results are obtained from spin torqueferromagnetic resonance experiments on CoFeB/Ta and Pt/Permalloy samplesseparately, under a series of driven frequencies.

FIG. 12 shows a schematic diagram of a magnetic data storage device thatstores information in magnetic domains and that uses the SHE as awriting mechanism. The magnetic moment for both the FM wire and thepinned layer of the MTJ are magnetized in-plane.

FIG. 13 is a variant of the structure shown in FIG. 12, which hasperpendicular to plane magnetic anisotropy.

FIG. 14 shows crystal structure, resistivity and stabilitycharacteristics of alpha-phase tungsten and beta-phase tungsten.

FIG. 15 shows resistivity of tungsten thin films as a function ofthickness using DC magnetron sputter deposition.

FIG. 16 shows the graphical results of measurements of a spin Hall anglein a 6 nanometer thick tungsten film.

FIG. 17 shows the results of spin Hall angle measurements of threedifferent tungsten films from FMR data and one tungsten film fromswitching data.

FIG. 18 shows a schematic diagram of a three terminal spin Hall effectdevice using a tungsten film as an SHE base layer.

FIG. 19 shows at left a schematic diagram of a spin Hall effect memoryelement, and at right spin Hall effect switching behavior at roomtemperature for a three terminal spin Hall effect device that utilizes a6 nanometer thick tungsten film layer.

FIG. 20 shows at left device parameters, and at right spin Hall effectswitching behavior at room temperature for a three terminal spin Halleffect device that utilizes a high resistivity 5.2 nanometer thicktungsten film layer.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments include a spin torque magnetic random access memory(ST-MRAM) structure, a method for fabricating the ST-MRAM structure anda method for operating a ST-MRAM device that results from the ST-MRAMstructure. The ST-MRAM structure and device in accordance with theembodiments use a spin Hall effect base layer located and formedcontacting a free layer within the ST-MRAM structure and device so thata lateral current through the SHE base layer may be used to switch amagnetic orientation of a free layer with respect to a pinned layerwhile measuring or applying a sense voltage and a sense currentperpendicularly through a magnetoresistive element stack that comprisesthe pinned layer, a non-magnetic spacer layer and the free layer thatcontacts the spin Hall effect base layer.

The embodiments contemplate that specific materials may be used for thespin Hall effect base layer, such as but not limited to Ta and W, withthe high-resistivity beta forms of Ta and W being particularly desirablewithin the context of particular materials characteristics. Additionalcandidate materials that may comprise the spin Hall effect base layerinclude, but are not limited to, Pt, Pd, Nb, Mo, Ru, Re, Os, Ir, Au, Tl,Pb, Bi as well as the alloys based upon those transition metals such asCu_(1-x)Bi_(x), Ag_(1-x)Bi_(x), Cu_(1-x)Ir_(x), Ag_(1-x)Ir_(x),Cu_(1-x)W_(x), Ag_(1-x)W_(x), Cu_(1-x)Ta_(x), Ag_(1-x)Ta_(x), and highresistivity compounds that incorporate one or more elements with highatomic number, such as compounds with the A15 crystal structure such asTa₃Al, Nb₃Sn, W₃Ge, and other compounds such as TaN, WN and NbN.

I. Exemplary Selection Criteria for a Non-Magnetic Conductor Materialfor a Spin Hall Effect Base Layer within an ST-MRAM Structure and Devicein Accordance with the Embodiments

The embodiments relate to ST-MRAM structures and devices generally inaccordance with FIG. 1, FIG. 2, FIG. 3, FIG. 10, FIG. 12 and FIG. 13,which utilize a spin Hall effect base layer which contacts a free layerwithin an ST-MRAM structure. As described above, the ST-MRAM structureand related device provides for a lateral switching current within thespin Hall effect base layer for purposes of effecting a magnetic spintransition within the free layer that contacts the spin Hall effect baselayer. A sensing voltage or a sensing current may then be utilized toread separately and perpendicularly though either a magnetoresistivetunneling junction, or a magnetic spin valve, that comprisessequentially layered a pinned layer, a non-magnetic spacer layer and thefree layer that contacts the spin Hall effect base layer, to determine a0 data state or a 1 data state within the magnetoresistive tunnelingjunction.

In general, the spin Hall base layer in accordance with the embodimentsmay comprise any of several non-magnetic conductor materials, includingbut not limited to tantalum and tungsten, although other non-magneticconductor materials as pure metals and metal alloys and compounds arenot precluded within the context of the embodiments, as furtherdescribed above.

A first characteristic of the non-magnetic conductor material from whichis comprised the spin Hall effect base layer is a relatively large spinHall angle; values greater than 0.05, and more preferably greater thanabout 0.1, will generally be needed for practicable devices. The spinHall angle denotes a conversion efficiency between a charge currentdensity and a spin current density within the non-magnetic conductormaterial that comprises the spin Hall effect base layer. As noted indetail below, this is an intrinsic property of conducting materials andthus should be measured for each specific material using the spin torqueFMR technique or direct switching experiments.

Moreover, when selecting a non-magnetic conductor material for the spinHall effect base layer, the embodiments contemplate that such anon-magnetic conductor material be selected to have a thickness nogreater than about 5 times the spin diffusion length for thenon-magnetic conductor material that comprises the spin Hall effect baselayer. The spin diffusion length denotes the length scale over which anon-equilibrium spin polarization of the conduction electrons in anon-magnetic metal decays exponentially to zero by spin flip scatteringprocesses. Such a spin diffusion length can be measured, e.g., by usingthe spin torque FMR technique with a series of samples having differentthicknesses for the spill Hall effect base layer. Such a maximumthickness is governed by the consideration that the switching currentincreases linearly with the thickness of the SHE base layer when thisthickness is much larger than its spin diffusion length.

Additional considerations for selecting a non-magnetic conductormaterial for the spin Hall effect base layer will differ depending onwhether the equilibrium magnetization orientation of the magnetic freelayer is in the sample plane or perpendicular to this plane. If the freelayer magnetization is in-plane, the spin Hall torque can drive magneticswitching by modifying the effective magnetic damping of the free layer.To achieve this result, the spin-Hall material should be chosen so thatin the absence of current it does not increase the magnetic damping ofthe free layer by more than a factor of two above its intrinsic value.Ta and W satisfy this criterion, but other materials with strongspin-orbit coupling such as Pt do not. If the free layer magnetizationis perpendicular to the sample plane, the spin Hall torque can achieveswitching by overcoming a torque due to the perpendicular anisotropy. Toachieve this result, the spin-Hall materials should be chosen so that itcontributes to the perpendicular magnetic anisotropy of the free layerto provide a total magnetic anisotropy energy in the range between 40k_(B)T and 300 k_(B)T, so that the magnetic free layer has thermalstability but can still be switched at low values of applied current. W,Ta, and Pt can all satisfy these criteria.

In concert with the above materials selection considerations for anon-magnetic conductor material from which may be comprised the spinHall base layer within an ST-MRAM structure and device in accordancewith the embodiments, the data for candidate materials platinum,tantalum and tungsten is illustrated as follows in Table I.

TABLE I Metal SD Length SHE Angle W ~1 nm 0.2-0.3 Ta ~1 nm 0.15 Pt 1.4nm  0.06II. General Considerations Related to ST-MRAM Structures and Devices inAccordance with the Embodiments

The embodiments utilize the spin Hall effect as a writing mechanism fora practical three terminal ST-MRAM device. To illustrate the basicconcept, FIG. 1 in particular shows a schematic cross-sectional diagramof the three terminal ST-MRAM device structure, where the SHE isemployed as the writing mechanism and an MTJ structure is employed toread out the data which is stored via the magnetic orientation of a freelayer FL within a magnetic tunnel junction with respect to a pinnedlayer within the magnetic tunnel junction.

Within the ST-MRAM in accordance with the embodiments, the magnetictunnel junction comprises a pillar-shaped magnetoresistive element andcomponent with a lateral dimension generally in the sub-micron ornanometer range (i.e., from about 10 nanometer to about 500 nanometer).A free ferromagnetic layer with a magnetic moment {right arrow over(m)}₁ is made of soft ferromagnetic material with small to mediumcoercive field, generally in a range from about 10 to about 5000Oersted. Typical thicknesses for the free layer range from 0.5 nanometerto about 3 nanometers. A pinned ferromagnetic layer with magnetic moment{right arrow over (m)}₂ is made of soft or hard ferromagnetic materialwith a large coercive field, generally in a range from about 100 toabout 20000 Oersted, or pinned by additional antiferromagnetic layers.Typical thicknesses for the pinned magnetic layers range from 4nanometer to about 50 nanometers. The FL and the PL are separated by anon-magnetic spacer layer, and the non-magnetic spacer layer iscomprised of an insulating oxide material, or alternatively anon-magnetic metal such as Cu or Ag. A thickness of the non-magneticspacer layer usually ranges from about 0.5 nanometer to about 50nanometers. Typical materials for the magnetic layers may include (butare not limited to) Fe, Co, Ni, alloys of these elements, such asNi_(1-x)Fe_(x), alloys of these elements with a non-magnetic material,such as Fe_(1-x)Pt_(x) and Co_(x)Fe_(y)B_(1-(x+y)), and ferromagneticmultilayers made from those materials, such as (Co/Ni)_(n), (Co/Pt)_(n),and (Co/Pd) where n represents the repeat number of the multilayer.Typical oxide materials for the non-magnetic spacer layer may include(but are not limited to) magnesium oxide (MgO), boron doped magnesiumoxide (Mg(B)O), stoichiometric and non-stoichiometric aluminum oxide(Al₂O₃ and AlO_(x)), titanium oxide, tantalum oxide, hafnium oxide,boron nitride and silicon oxide.

In contact with the FL of the magnetic tunnel junction is a non-magneticthin-film strip made of one of a variety of possible materials thatexhibit a strong spin Hall effect (SHE) (i.e., a spin Hall effect baselayer). The non-magnetic strip is patterned into nanometer scale ormicrometer scale wires. Typical materials with strong SHE may include(but are not limited to) Ta, W, Pt, alloys and compounds thatincorporate these or other high atomic number elements, and alloys suchas Cu_(1-x)Ir_(x), Cu_(1-x)Bi_(x).

As is shown in FIG. 1, three terminals of the ST-MRAM structure areformed where electrical connections may be made to the structure. Oneterminal is on the pillar, close to the PL of the MTJ, and the other twoterminals are the two ends of the non-magnetic strip that comprises thespin Hall effect base layer. Writing current is applied between the twoterminals on the non-magnetic strip while reading current is appliedbetween the terminal on the pillar and either one of the two terminalson the non-magnetic strip that comprises the SHE base layer.

The schematic perspective view diagram shown in FIG. 1 gives therelative location of each layer but does not necessarily reflect theirexact positions. For example, the FL of the MTJ can be either at thebottom of the pillar, as shown in FIG. 1 or on the top of the pillar, asis in FIG. 2. But in any case the non-magnetic strip with the strong SHEis always adjacent to (and typically adjoining) the FL. When the FL isat the bottom, the non-magnetic strip is also at the bottom of thedevice structure, next to a substrate (i.e., which is not illustrated)over which is located and formed all of the material layers within theST-MRAM in accordance with the embodiments. When the FL is on the top,the PL is located and formed on the substrate side of the tunnelbarrier, the FL is above the tunnel barrier, and the non-magnetic stripthat comprises the SHE base layer is located on the top of the ST-MRAMdevice.

The equilibrium orientation of magnetic moments of the FL and PL {rightarrow over (m)}₁, {right arrow over (m)}₂ can be either in the filmplane as is illustrated in FIG. 1, or perpendicular to the film plane asis illustrated in FIG. 3. The injected spins from the SHE are alwaysoriented in the film plane and perpendicular to the flowing direction ofthe current in the non-magnetic strip, as illustrated in FIG. 4. Theorientation of the injected spins is determined by the formula {rightarrow over (J)}_(s)∝θ_(SH){right arrow over (σ)}×{right arrow over(J)}_(c). Here, {right arrow over (σ)} is the direction of the injectedspin moments (not the angular momenta) and θ_(SH) is the spin Hallangle, which is a parameter intrinsic to each material and quantifiesthe magnitude of the SHE in that specific material. Since the FL iseither above or under the non-magnetic strip carrying the charge currentJ_(c), the spin current {right arrow over (J)}_(s) is either flowingalong the +z or −z direction in FIG. 4. Therefore, according to theformula above, the injected spin {right arrow over (σ)} is either alongthe +x or −x direction, depending on the direction of J_(c) and the signof the spin Hall angle θ_(SH).

When {right arrow over (m)}₁ and {right arrow over (m)}₂ are in plane,and perpendicular to the current direction mentioned above (i.e. along+/−x axis direction). {right arrow over (m)}₁ is collinear (eitherparallel or anti-parallel) with the injected spins from the SHE {rightarrow over (σ)}. In this case the injected spins act as an effectivemagnetic damping which depending upon the orientation of the spin can beof either sign, i.e. either positive or negative damping. Under thisconfiguration, the SHE induced switching works in the same way as theconventional spin torque induced switching. Within a spin Hall effectST-MRAM device in accordance with the embodiments, the spin current isgenerated using a non-magnetic material instead of a ferromagneticmaterial layer, such as but not limited to a ferromagnetic polarizerlayer. When {right arrow over (m)}₁ is parallel with {right arrow over(σ)}, the spin current will make the current magnetization orientationmore stable, and will not cause switching. Otherwise, when {right arrowover (m)}₁ is antiparallel with {right arrow over (σ)}, if the spincurrent is large enough, the magnetization of FL will be switched.Therefore, currents with opposite sign will inject spins with oppositeorientation into the FL, and those opposite orientations will result indifferent preferable orientations of the FL magnetization, so areversible deterministic switching can be realized by determining thedirection of the electrical current through the SHE generating layerthat is designated as the SHE base layer.

Since the current required for switching of the FL when the FL and PLare magnetized in the plane of the films is linearly proportional to theeffective magnetic damping of the free layer, the most efficientswitching will occur when this effective damping is minimized. Thus itis highly preferable that the normal layer in which the SHE is generatedbe some material other than Pt. This is because by the process know asspin-pumping (S. Mizukami, Y. Ando and T. Miyazaki, J. Magn. Magn.Mater. 239, 42 (2002), Y. Tserkovnyak, A. Brataas, and G. E. W. Bauer,Phys. Rev. Lett. 117601 (2002)), whereby the magnetic damping of a FLwhen in electrical contact with a Pt layer is greatly enhanced by spinsdiffusing across the interface from the FL into the Pt where the spinquickly relax. If for example Ta or W is employed to generate the SHErather than Pt, the spin pumping effect is much less. FIG. 5 shows themagnetic damping of a ferromagnetic layer of CoFeB in contact with athin layer of Pt as determined by ferromagnetic resonance and forcomparison that of two similar bilayers, one consisting of aferromagnetic film of CoFeB in contact with a Ta film, and the other aferromagnetic film of CoFeB in contact with a W film. The magneticdamping of the CoFeB/Ta and CoFe/W bilayers is less than that of theCoFeB/Pt bilayer by approximately a factor of two or more, demonstratingthe advantage of the use of Ta or W, or another SHE material with lowspin relaxation rates, other than Pt for spin torque switching via ananti-damping spin torque.

The giant SHE in Ta, together with its small effect on the damping ofadjacent magnetic layers, makes Ta an excellent material for effectingspin torque switching of an in-plane magnetized free layer in a magnetictunnel junction. In conventional anti-damping spin torque switchingwhere the spins are injected either nearly parallel or anti-parallel tothe initial orientation of the local magnetic moment, the criticalcurrent density J_(C0) for switching in the absence of thermalfluctuations can be approximately calculated as

${J_{C\; 0} \approx {\frac{2e}{\hslash}\mu_{0}M_{S}t\;{{\alpha\left( {H_{C} + {M_{eff}/2}} \right)}/\left( {J_{S}/J_{e}} \right)}}},$where M_(S), t, and H_(C) represent the saturation magnetization, thethickness and the coercive field of the free layer, respectively.

To demonstrate in-plane magnetic switching induced by the spin Halleffect in Ta, a three-terminal device was fabricated, consisting of themultilayer: substrate/Ta(6.2)/CoFeB(1.6)/MgO(1.6)/CoFeB(3.8)/Ta(5)/Ru(5)(thicknesses in nm) patterned into the geometry illustrated in FTG. 6.The Ta bottom layer was patterned into a 1 μm wide and 5 μm long strip(with resistance 3 kΩ) and the rest of the layers were etched to form amagnetic tunnel junction (MTJ) nanopillar on top of the Ta with lateraldimensions ˜100×350 nm, and with the long axis of the nanopillarperpendicular to the long axis of the Ta micro strip.

For the spin torque switching measurement a DC current I_(Ta) wasapplied to flow through the Ta microstrip while the differentialresistance dV/dI of the magnetic tunnel junction was monitored. FIG. 7shows the abrupt hysteretic switching of the MTJ resistance thatoccurred when I_(Ta) was swept through 1 mA, which resulted inantiparallel to parallel (AP-P) switching. This switching was reversed(P-AP switching) when the current was swept back past −1 mA. Note thatduring the spin-torque switching measurement a −3.5 mT in-plane magneticfield was applied along the long axis of the MTJ to cancel the magneticdipole field from the top layer of the MTJ acting on the bottom layer.This biased the free layer of the junction at the midpoint of itsmagnetoresistance loop. In an SHE device that is optimized for memorycell application the top pinned layer will be, for example, a syntheticanti-ferromagnetic tri-layer that will be balanced to result in the netdipole field at the FL being close to zero, which will remove the needfor the external in-plane field.

This 3-terminal SHE switching result demonstrates that a spin Halleffect as strong as that in beta-Ta can be very effective in switchingthe free layer of a magnetic tunnel junction without any substantialpart of the switching current required to flow through the MTJ, whichsolves a major reliability issue associated with conventional 2-terminalST-MRAM devices. Moreover this result demonstrates the important featureof this embodiment that the current required for switching the FL frombeing parallel (P) to being antiparallel (AP) to the PL is essentiallythe same as is required for switching in the opposite direction, AP toP, and of course the electrical impedance for the write operation is thesame for both switching directions. This is in sharp contrast to thecase of the 2-terminal MTJ spin torque device where the switchingcurrents are quite different for the two switching directions, and theelectrical impedance at the beginning of the write operation is alsoquite different for the two switching directions. These symmetriccharacteristics of the write operation in a 3-terminal SHE switched MTJmemory cell provide advantages for the design of magnetic memorycircuits.

The SHE induced switching can also be realized with {right arrow over(m)}₁ and {right arrow over (m)}₂ oriented perpendicular to plane. Inthis configuration, the injected spins from the SHE {right arrow over(σ)} are still along +/−x-axis while the equilibrium position for {rightarrow over (m)}₁ is aligned along the +/−z axis. So, the direction of{right arrow over (m)}₁ and that of {right arrow over (σ)} areperpendicular to each other and effect of the injected spins is nolonger equivalent to an effective damping. Instead the effect of thespin torque can be described using an effective magnetic field B_(ST).The spin torque per unit moment generated by injected spin current canbe written as

${{\overset{->}{\tau}}_{ST} = {\frac{h}{2{eM}_{S}t}{J_{S}\left( {\hat{m} \times \hat{\sigma} \times \hat{m}} \right)}}},$where ℏ, e, M_(S) and t represent the Planck's constant, electroncharge, saturation magnetization of the FL and the thickness of the FL,respectively, and J_(S) is the spin current injected into the FL fromthe SHE. Meanwhile, the torque generated by a magnetic field in generalcan be written as {right arrow over (τ)}={circumflex over (m)}×{rightarrow over (B)}. By comparing the form of the two torques, the effectivemagnetic field induced by the spin Hall effect has the form

${\overset{->}{B}}_{ST} = {{- \frac{h}{2{eM}_{S}t}}J_{S}\hat{\sigma} \times {\hat{m}.}}$Therefore, {right arrow over (B)}_(ST) is always perpendicular to {rightarrow over (m)}₁ and points clockwise or counterclockwise, dependingupon the direction of the injected spins. FIG. 8A gives an illustrationon the direction of {right arrow over (B)}_(ST) when the injected spin{right arrow over (σ)} is along the −x direction. If J_(S) is largeenough such that |{right arrow over (B)}_(ST)|>0.5B_(an) ⁰, where B_(an)⁰ is the maximum anisotropy field that the magnetic film can provide,then {right arrow over (B)}_(ST) will induce a continuous rotation of{right arrow over (m)}₁. In a multi-domain ferromagnetic layer, wherethe coercive field of the magnetic film B_(c) is smaller than B_(an) ⁰,the corresponding requirement on {right arrow over (B)}_(ST) can beloosened to approximately |{right arrow over (B)}_(ST)|>0.5B_(c). Underthe effect of {right arrow over (B)}_(ST), {right arrow over (m)}₁ willbe switched continuously, without a deterministic final state. So anexternal magnetic field has to be introduced in order to get adeterministic switching. In FIG. 8B, an external field in the +ydirection is applied as an example. Using {right arrow over (m)}_(z) torepresent the z component of {right arrow over (m)}₁, it can be seenthat the state with m_(z)>0 will become a stable state because {rightarrow over (B)}_(ST) and {right arrow over (B)}_(ext) can be balancedout with each other while m_(z)<0 states are still non-stable because{right arrow over (B)}_(ST) and {right arrow over (B)}_(ext) works inthe same direction, causing {right arrow over (m)}₁ to continue torotate. Therefore, under an applied field in the +y direction, spinsinjected in the −x direction can switch {right arrow over (m)}₁ into them₂>0 state. By reversing the writing current direction, spins from theSHE will be injected along +x direction, causing {right arrow over (m)}₁to be switched into the m_(z)<0 state. In conclusion, by using spinsinjected from the SHE, a reversible deterministic switching can berealized. The role of the external magnetic field {right arrow over(B)}_(ext) is to break the symmetry of the system and to get a definitefinal state. The magnitude of this field can be as small as a fewmilli-Telsa (mT) as is demonstrated in the experiment. FIG. 9 shows theexperimental result, which utilizes the spin current from the SHE toswitch the magnetic moment at room temperature. The sample was formed bya 20 um wide, 2 nm thick Pt strip and a 0.7 nm Co magnetic layer incontact with the Pt strip, which has a perpendicular magneticanisotropy, 1.6 nm of Al is utilized as the capping layer to protect theCo from oxidation by the atmosphere. The anomalous Hall effect is usedto monitor the magnetization orientation of the Co layer. The x-axis inFIG. 9 represents the applied current in the Pt strip and the y-axisreflects the anomalous Hall resistance under corresponding current. Itcan be seen that under an external field of +10 mT, the magnetic momentof Co layer can be switched back and forth with opposite appliedcurrent. In the device configuration for an MRAM cells, in order toprovide the required external field along the current flowing direction,an in-plane magnetized fixed magnetic layer of a few nanometersthickness can be added onto the top of the MTJ, as is shown in FIG. 10.The dipole field generated by this in-plane magnetic layer will give thecurrent induced-switching a deterministic final state. Otherconfigurations of ferromagnetic thin films can also be employed togenerate this small external in-plane magnetic field.

While the damping enhancement or spin pumping effect of a Pt layer incontact with the FL does not cause an increase in the required switchingcurrent when the FL and PL are magnetized perpendicular to the plane ofthe film layers, in that configuration it still be preferable to use Ta,W, or some material other than Pt to generate the spin current by thespin Hall effect. That is because the efficiency of the SHE in Ta and Wis much higher than that of Pt in generating a spin current from anapplied electrical current. This is demonstrated in FIG. 11, where theratios of the spin current density to the electrical current density areplotted for a NiFe/Pt bilayer and for a CoFe/Ta bilayer. The spin Hallefficiency for the Ta case is more than twice that for the Pt case andthat for W in its beta phase is approximately twice that for Ta.

The writing operation can be realized by applying currents in thenon-magnetic strip for magnetic tunnel junctions with both in-plane andperpendicular to plane magnetic anisotropy, as is discussed above. Thereading operation should be similar with conventional ST-MRAM device. Asensing current across the insulating barrier will yield a differentvoltage signal, depending upon the relative orientation of the FL andPL.

Compared with the conventional ST-MRAM, one advantage of the MRAM cellemploying the SHE as the writing mechanism is stronger torques per unitcurrent. The SHE can provide a torque corresponding to a transfer orangular momentum greater than 1 unit of spin (h/2pi) per electron in theapplied current, where the torque in conventional ST-MRAM must always beweaker than 1 unit of transferred spin (h/2pi) per electron in thecurrent. A second advantage of the MRAM cell employing the SHE as thewriting mechanism lies in that the writing current no longer passesthrough the tunnel barrier, which can greatly increase the lifetime ofthe memory cell and greatly eases the reproducibility margins needed toachieve reliable reading and writing. In conventional ST-MRAM, sinceboth the writing and reading operation rely on the tunneling barrier, anundesirable trade-off has to be made in order to get a large tunnelingmagnetoresistance and at the same time allow for a large current to flowthrough the barrier. In many cases, the requirement of those two cannotbe satisfied simultaneously. In contrast, in the three-terminal MRAMcell that uses the SHE as the writing mechanism, the performance of theMTJ can be optimized just for the reading operation. Therefore one gainsconsiderable freedom in the design of the MTJ, for example, thethickness of the tunnel barrier can be adjusted to get the optimumtunneling magnetoresistance and the appropriate impedance match with thecircuitry that provides the write currents and the circuitry that readsthe sensing voltage.

An ST-MRAM in accordance with the embodiments has an advantage ofsimplicity of fabrication. The ST-MRAM in accordance with theembodiments separates the writing current and the reading currentwithout adding significant complexities into a fabrication process. Onlya single nanometer scale pillar is fabricated in the ST-MRAM structureshown in FIGS. 1-3.

In the ST-MRAM device geometries discussed thus far in FIGS. 1-3 andFIG. 10, the thin film magnetic free layer is of limited lateral extentand hence contains (approximately) a single magnetic domain, whose twopossible net magnetization directions are indicated by the arrows in therelevant FIGS. 1-3 and FIG. 10. In another category of ST-MRAM magneticmemory device in accordance with the embodiments, the magnetic freelayer consists of a long wire or wire segment in which information isstored via the positions of magnetic domain walls which separate domainswith different magnetization directions.

To that end, the embodiments also propose to use the torque from the SHEto enhance the ability of an electrical current to manipulate thepositions of magnetic domain walls by using samples in which themagnetic free layer wire is in contact with a non-magnetic thin filmthat exhibits a strong SHE, in combination with a pinned magnetic layerto read out the magnetic orientation of the free layer, as illustratedin FIG. 12 and FIG. 13. The current may flow laterally parallel to thesample plane or a lateral current may be applied in combination with avertical current. The torque from the SHE may directly assist in movingthe domain wall and it may also stabilize the configuration of thedomain wall enabling it to be moved at greater velocity and improvedefficiency compared to the influence of the conventional spin transfertorque effect alone.

III. Specific Materials Considerations Related to the Spin Hall Effectin the ST-MRAM Device Structures in Accordance with the Embodiments

In accordance with section I., above, there are at least three materialsparameters related to the materials properties of the spin Hall effectbase layer within which the spin Hall effect (SHE) occurs that can beoptimized for efficient switching of a free layer magnetic element bythe spin torque exerted by the SHE generated spin current.

First, the conversion efficiency from the charge current to spin currentdepends on the spin Hall angle, θ_(SII)=J_(S)(∞)/J_(e), which is theratio of the generated transverse spin current density to the appliedlongitudinal electrical current density. This is an intrinsic propertyof conducting materials, which varies from material to material and as afunction of materials quality for a given material. A large spin Hallangle corresponds to large spin current generation efficiency. Forexample, W, Ta, and Pt have quite large spin Hall angles. Other metallicelements or alloys have the potential to have a large spin Hall angle.Conducting films formed from metallic elements, alloys, and compoundsthat incorporate high atomic number elements, and conducting films dopedwith high atomic number dopants with strong spin-orbit scattering suchas Au doped with Pt or Cu doped with Ir or Bi may also have a large spinHall angle.

A second parameter is the spin diffusion length λ_(5F) of thenonmagnetic material that is used to generate the transverse spincurrent via the SHE. The spin diffusion length is the length scale overwhich a non-equilibrium spin density relaxes back to equilibrium withinthe material. The spin current generation efficiency is optimized whenthe spin diffusion length is comparable to or smaller than the thicknessof the non-magnetic thin film nanowire d in the schematic drawings ofthe device (see, e.g., FIG. 1. FIG. 3 and FIG. 10), according to theequation [J_(S)(∞)/J_(e,SHM)][1−sech(d/λ_(SF))]. So, for the SHE writingmechanism to work with the highest efficiency, one will typically desirea spin diffusion length as small as possible. One approach is to utilizea highly resistive metal with a high spin Hall angle and a short elasticmean free path l for its conduction electrons as the spin diffusionlength scale directly with l^(1/2).

A large spin Hall angle and short spin diffusion length are beneficialfor an MRAM cell with the magnetic moment of the free, reversible,layer, lying either in-plane or perpendicular-to-the-plane, the twocases demonstrated in FIG. 1 and FIG. 3 respectively. For the lattercase of the free layer magnetized perpendicular to the plane, theelectrical, or charge, the minimum current required to initiate the spintorque switching of a thermally stable (anisotropy energy=40 k_(B)T)perpendicularly polarized magnet is given approximately by

$I_{c} = {\frac{2{{e\left( {40k_{B}T} \right)}\left\lbrack {d + {\left( {\sigma_{F}/\sigma_{SHM}} \right)t}} \right\rbrack}}{\hslash\;{{L\left( {{J_{S}\left( {d = \infty} \right)}/J_{e,{SHM}}} \right)}\left\lbrack {1 - {{scch}\left( {d/\lambda_{sf}} \right)}} \right\rbrack}}{\frac{{M_{S}\left( {I_{c}} \right)}{B_{an}^{0}\left( {I_{c}} \right)}}{{M_{S}\left( {I = 0} \right)}{B_{an}^{0}\left( {I = 0} \right)}}.}}$

Here J_(S)(d=∞)/J_(e,SHM) slat is the spin Hall angle, d is thethickness of the non-magnetic metal layer M within which the SHE acts togenerate the transverse spin current, t is the thickness of the freelayer magnet, L is the length of the free layer magnet in the directionof the electrical current flow, k_(B) is the Boltzmann constant, T isthe absolute temperature of the device, σ_(F) is the electricalconductivity of the free layer magnet, σ_(SHM) is the conductivity ofthe non-magnetic layer, λ_(sj) is the spin diffusion length within thelayer generating the spin Hall effect, M_(S)(|I_(c)|) is the saturationmagnetization of the free layer magnet which may vary with current dueto ohmic heating, and B_(an) ⁰ is the effective anisotropy field of theperpendicularly-polarized free layer. The anisotropy field B_(an) ⁰should be just large enough to ensure thermal stability of the freelayer ferromagnet. The switching current is clearly minimized when thespin Hall angle is maximized and the spin diffusion length minimized.

A third material parameter of importance to the optimization of thewriting of data in an ST-MRAM cell via the SHE is the degree to whichthe presence of the spin Hall metal will increase the magnetic dampingcoefficient (α) of an adjacent magnetic layer, e.g., through thespin-pumping effect. Minimizing this increased damping is required inorder to minimize the electrical current necessary to effect themagnetic reversal of the free layer ferromagnet in MRAM devices wherethe magnetic moment is magnetized in the film plane, as is the caseshown in FIG. 1. Some materials with a large spin Hall angle, such asPt, induce a large amount of unwanted excess damping, while othermaterials with a large spin Hall angle, such as W and Ta are preferablebecause they add much less damping. The minimum current needed to switcha thermally-stable in-plane magnetized free layer is given approximatelyby:

$I_{c} \approx {\frac{2e}{\hslash}\alpha\frac{d}{L}{\frac{\left\lbrack {{\frac{{M_{S}\left( I_{c} \right)}{H_{k}\left( I_{c} \right)}}{{M_{S}(0)}{H_{k}(0)}}2\left( {40k_{B}T} \right)} + \frac{{\mu_{0}\left\lbrack {{M_{S}\left( I_{c} \right)}{Vol}} \right\rbrack}{M_{eff}\left( I_{c} \right)}}{2}} \right\rbrack\left\lbrack {1 + {\frac{\sigma_{F}}{\sigma_{SHM}}\frac{t}{d}}} \right\rbrack}{\left\lbrack {{J_{S}\left( {d = \infty} \right)}/J_{e,{SHM}}} \right\rbrack\left\lbrack {1 - {{scch}\left( {d/\lambda_{SF}} \right)}} \right\rbrack}.}}$

Here Vol is the volume of the free layer magnet, μ₀ is the permeabilityof free space, H_(K)(I) is the in-plane coercive field of the free layermagnet and M_(eff)(I) is effective demagnetization field of the freelayer magnet, both of which may vary with current due to heatingeffects. Thus for the case of MRAM cells with in-plane magnetized freelayers there is direct benefit of enhanced efficiency for SHE inducedswitching by utilizing a material to generate the transverse spincurrent that has a large as possible spin Hall angle, as short aspossible spin diffusion length, and a low value of the damping parameterα for the magnetic free layer in contact with the spin Hall metal. Withmaterials whose properties, including the spin Hall angle, have alreadybeen characterized it is straightforward to calculate that efficiency ofmagnetic reversal by the SHE can be quite comparable to best valuesachievable with conventional spin torque switching. Together with thisexcellent efficiency, the greater reliability and ease of fabrication ofa SHE ST-MRAM device makes it substantially superior for highperformance, non-volatile magnetic memory applications.

IV. Additional Considerations Related to Tungsten as an SHE Base Layerin ST-MRAM Device Structures in Accordance with the Embodiments

As discussed above the spin Hall effect that arises from the currentthat flows in a thin normal metal strip of the proper type canefficiently effect the magnetic excitation and reversal of polarity of asmall nanoscale ferromagnet that is placed adjacent to, and inelectrical contact with the normal metal strip. While not being bound byany particular theory of operation of the embodiments, it is believedthat this is due to the spin-orbit interaction of the conductionelectrons in metals with high atomic number Z, electrons having one spinorientation are deflected preferentially in one direction that istransverse to the direction of electron current flow and those electronswith their spin orientation in the opposite orientation are deflected inthe opposite transverse direction. The result is a diffusion ofspin-polarized electrons to the two opposing surfaces of the normalmetal strip, which is known as the spin Hall effect. This “spin current”when it impinges onto the surface of the nanomagnet placed on thesurface of the film microstrip can exert a spin torque on the nanomagnetthrough the now well known phenomena of spin transfer. This occursbecause the quantum mechanical angular momentum of the electronsentering or reflected from the nanomagnet that is transverse to thelocal moment of the nanomagnet has to be absorbed by that moment.

Discussed above is also how the spin Hall effect in Pt and in Ta thinfilms can be strong enough to effect magnetic reversal in a mannersuitable for application in nanoscale magnetic memory cells. In generalthe more efficient the conversion of electrical current density totransverse spin current density the better the overall effectiveness ofthe device. The ratio of the transverse spin current density J_(s) tothe longitudinal electrical current density J_(e) is known as the spinHall angle θ=J_(e)/J_(e). Equivalently one may describe the spin Hallangle as θ=σ_(s)/σ_(e), where σ_(s) is the transverse spin conductivityand σ_(e) is the longitudinal electrical conductivity. In accordancewith above disclosure the spin Hall angle for high resistivity Ta thinfilm microstrip in the so-called beta phase can be ≧0.15.

To maximize θ it is necessary to utilize a material with a strong spinorbit interaction. In the case of a metallic system where the electronmean-free-path between elastic collisions in the metal in question isquite short the spin Hall effect is expected to be in the so-calledintrinsic regime. In that case the electrical conductivity σ_(e)decreases linearly with electron mean-free-path but the transfer spinconductivity σ_(s), which depends only on the interaction of theconduction electron with the atomic electron orbitals about the high Zatoms in the metal is independent of the mean free path. Thus θ can bequite large if there is a large spin-orbit interaction in a lowconductivity metal.

As an example, one may determine that the spin Hall angle in tungsten Wthin films that have been produced in a manner such as to be at leastpartially in the beta-W phase, which is generally believed to have theA15 crystal structure and is relatively highly resistive, can be oforder 0.3 and possibly higher. The alpha-W phase is in thebody-centered-cubic crystal structure and is of considerably lowerresistivity. See, e.g. FIG. 14. The beta-W phase is stable in very thin,thickness ≦10 nm, W layers may be produced by magnetron sputtering inhigh vacuum and also by other means. Depending upon the method ofproduction, thickness and post-deposition processing the W film can benearly 100% in the beta phase, in a mixed alpha and beta phase or in thepure alpha phase, with the resistivity ranging from over 200 microOhm-cmin the beta phase to less than 30 microOhm-cm in the alpha phase. Anexample of the variation in resistivity with thickness and processing isshown in FIG. 15.

To determine the strength of the spin Hall effect in a given material,the most straightforward approach is to produce a thin microstrip layerof the materials in question and then deposit a thin ferromagnetic layeron top. One may then pass a microwave current through the bilayer in thepresence of a variable applied magnetic field. As the field amplitude isvaried at the proper combination of frequency and field a ferromagneticresonance (FMR) excitation can be generated within the ferromagneticmaterial, which can be detected via the anisotropic magnetoresistanceeffect. There are two ways the resonance can be generated. The first isdue to the magnetic field generated by the portion of the current thatflows in the spin Hall material, the second by the transverse spincurrent from the spin Hall effect that generates a spin torque, via thespin transfer effect, on the ferromagnetic layer when the spin currentimpinges on it. The symmetry of the FMR response as the function offield is antisymmetric about the resonance field for the magnetic fieldresponse but is symmetric from the spin Hall effect. This allows adirect calibration of the strength of the SHE. In addition anindependent measure of the strength of the spin Hall effect is todetermine the linewidth of the FMR signal as the function of a dccurrent flowing through the bilayer microstrip. Depending on thedirection of the electrical current flow, the SHE either linearlyincreases the effective damping of the ferromagnetic resonance, hencenarrowing or broadening the linewidth as the function of field. This canprovide a second method of determining the spin Hall angle. An exampleof the results of such measurements with a W film is shown in FIG. 16.

Several W samples have been studied in this way. Depending upon theprocessing of the film and its thickness one may determine that the spinHall angle of a W material is quite large, and can be much larger thanfound with Ta. The results appear to vary at least linearly with the Wresistivity and perhaps more rapidly. See FIG. 17.

To demonstrate in practice the efficacy of the spin Hall effect in Wfilms to effect the writing, or magnetic reversal, of a free layerwithin an ST-MRAM magnetic memory cell one may fabricate a magnetictunnel junction on a thin W thin film strip, as shown schematically inFIG. 18. As illustrated in FIG. 19 and FIG. 20, depending on the Wsample and resistivity, direct electrical currents as small as 350microamps passing through the W strip can reverse the magneticorientation of the bottom ferromagnetic free layer electrode of themagnetic tunnel junction and hence switch the resistance of the tunneljunction from the low to high resistance state, or vice versa dependingon the direction of the current flown through the W strip. Still lowercurrents can be sufficient for reversing the magnetic orientation of theferromagnetic free layer of a magnetic tunnel junction if the width ofthe strip of spin Hall material is minimized to be close to that of thetunnel junction, and/or if the perpendicular magnetic anisotropy fieldH_(K) of the free layer is reduced to be of the order of 1000 Oe (0.1mTelsa) or lower.

All references, including publications, patent applications, and patentscited herein are hereby incorporated by reference in their entireties tothe extent allowed, and as if each reference was individually andspecifically indicated to be incorporated by reference and was set forthin its entirety herein.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. The term “connected” is to beconstrued as partly or wholly contained within, attached to, or joinedtogether, even if there is something intervening.

The recitation of ranges of values herein is merely intended to serve asa shorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wasindividually recited herein.

All methods described herein may be performed in any suitable orderunless otherwise indicated herein or otherwise clearly contradicted bycontext. The use of any and all examples, or exemplary language (e.g.,“such as”) provided herein, is intended merely to better illuminateembodiments of the invention and does not impose a limitation on thescope of the invention unless otherwise claimed.

No language in the specification should be construed as indicating anynon-claimed element as essential to the practice of the invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the spirit and scope of the invention. There isno intention to limit the invention to the specific form or formsdisclosed, but on the contrary, the intention is to cover allmodifications, alternative constructions, and equivalents falling withinthe spirit and scope of the invention, as defined in the appendedclaims. Thus, it is intended that the present invention cover themodifications and variations of this invention provided they come withinthe scope of the appended claims and their equivalents.

What is claimed is:
 1. A magnetic structure comprising: a spin Halleffect base layer located over a substrate; and a magnetic free layerlocated over the substrate and contacting the spin Hall effect baselayer, where a non-magnetic conductor material that comprises the spinHall effect base layer has: a spin Hall angle greater than about 0.05;and a thickness no greater than about 5 times a spin diffusion length inthe non-magnetic conductor material.
 2. The magnetic structure of claim1 wherein: the magnetic free layer is magnetically polarized in-planewith respect to the substrate; and a damping factor of the magnetic freelayer is increased by a factor less than about 2 by contact with thespin Hall effect base layer.
 3. The magnetic structure of claim 1wherein: the magnetic free layer is magnetically polarized perpendicularwith respect to the substrate; and a contribution to the perpendicularmagnetic anisotropy of the magnetic free layer is suitable to achieve atotal anisotropy energy of the magnetic free layer between 40 k_(B)T and300 k_(B)T by contact with the spin Hall effect base layer.
 4. Themagnetic structure of claim 1 wherein the non-magnetic conductormaterial comprises at least one of Pt, Pd, Nb, Mo, Ru, Re, Os, Ir, Au,Tl, Pb and Bi conductor materials, and alloys of the foregoing conductormaterials.
 5. The magnetic structure of claim 1 wherein the spin Halleffect base layer comprises at least one of a tantalum material and atungsten material.
 6. The magnetic structure of claim 1 wherein the spinHall effect base layer comprises at least in part at least one of a betaphase tungsten material and a beta phase tantalum material.
 7. Themagnetic structure of claim 1 wherein the spin Hall effect base layerincludes two terminals laterally separated by the free layer.
 8. Themagnetic structure of claim 1 further comprising: a non-magnetic spacerlayer located over the side of the free layer opposite the spin Halleffect base layer; a pinned layer located over the side of thenon-magnetic spacer layer opposite the free layer; and a single terminallayer located electrically connected to the pinned layer.
 9. Themagnetic structure of claim 8 further comprising: a second non-magneticspacer layer located interposed between the pinned layer and the singleterminal layer; and a second free layer located interposed between thesecond non-magnetic spacer layer and the single terminal layer.
 10. Themagnetic structure of claim 1 wherein the spin Hall effect base layer islocated closer to the substrate than the free layer.
 11. The magneticstructure of claim 1 wherein the free layer is located closer to thesubstrate than the spin Hall effect base layer.
 12. A magnetic structurecomprising: a spin Hall effect base layer located over a substrate andincluding two laterally separated terminals; a magnetoresistive stacklocated contacting the spin Hall effect base layer and comprising; amagnetic free layer located contacting the spin Hall effect base layer;a non-magnetic spacer layer located over the magnetic free layer; and apinned layer located over the non-magnetic spacer layer; and a thirdterminal electrically connected to the pinned layer, where anon-magnetic conductor material that comprises the spin Hall effect baselayer has: a spin Hall angle greater than about 0.05; and a thickness nogreater than about 5 times a spin diffusion length in the non-magneticconductor material.
 13. The magnetic structure of claim 12 wherein: themagnetic free layer is magnetically polarized in-plane with respect tothe substrate: and a damping factor of the magnetic free layer isincreased by a factor less than about 2 by contact with the spin Halleffect base layer.
 14. The magnetic structure of claim 12 wherein: themagnetic free layer is magnetically polarized perpendicular with respectto the substrate; and a contribution to the perpendicular magneticanisotropy of the magnetic free layer is suitable to achieve a totalanisotropy energy of the magnetic free layer between 40 k_(B)T and 300k_(B)T by contact with the spin Hall effect base layer.
 15. The magneticstructure of claim 12 wherein the spin Hall effect base layer comprisesat least one of Pt, Pd, Nb, Mo, Ru, Re, Os, Ir, Au, Tl, Pb and Biconductor materials, and alloys of the foregoing conductor materials.16. The magnetic structure of claim 12 wherein the spin Hall effect baselayer comprises at least one of a tantalum material and a tungstenmaterial.
 17. The magnetic structure of claim 12 wherein the spin Halleffect base layer comprises at least in part at least one of a betaphase tungsten material and a beta phase tantalum material.
 18. A methodfor fabricating a magnetic structure comprising: forming over asubstrate a spin Hall effect base layer comprising a non-magneticconductor material having: a spin Hall angle greater than about 0.05;and a thickness no greater than about 5 times a spin diffusion length inthe non-magnetic conductor material; and forming over the substrate andcontacting the spin Hall effect base layer a magnetic free layer. 19.The method of claim 18 wherein: the magnetic free layer is magneticallypolarized in-plane with respect to the substrate: and a damping factorof the magnetic free layer is increased by a factor less than about 2 bycontact with the spin Hall effect base layer.
 20. The method of claim 18wherein: the magnetic free layer is magnetically polarized perpendicularwith respect to the substrate; and a contribution to the perpendicularmagnetic anisotropy of the magnetic free layer is suitable to achieve atotal anisotropy energy of the magnetic free layer between 40 k_(B)T and300 k_(B)T by contact with the spin Hall effect base layer.
 21. Themethod of claim 18 wherein the non-magnetic conductor material comprisesat least one of Pt, Pd, Nb, Mo, Ru, Re, Os, Ir, Au, Tl, Pb and Biconductor materials, and alloys of the foregoing conductor materials.22. The method of claim 18 wherein the spin Hall effect base layercomprises at least one of a tungsten material and a tantalum material.23. The method of claim 18 wherein the spin Hall effect base layercomprises at least in part at least one of a beta phase tungstenmaterial and a beta phase tantalum material.
 24. A method for operatinga magnetic device comprising: providing a magnetic structure comprising:a spin Hall effect base layer located over a substrate and including twolaterally separated terminals; a magnetoresistive stack locatedcontacting the spin Hall effect base layer and comprising: a magneticfree layer located contacting the spin Hall effect base layer; anon-magnetic spacer layer located over the magnetic free layer; and apinned layer located over the non-magnetic spacer layer; and a thirdterminal electrically connected to the pinned layer, where anon-magnetic conductor material that comprises the spin Hall effect baselayer has: a spin Hall angle greater than about 0.05; and a thickness nogreater than about 5 times a spin diffusion length in the non-magneticconductor material; and applying a switching current to the twolaterally separated terminals to switch a magnetic direction of the freelayer with respect to the pinned layer.
 25. The method of claim 24wherein: the free layer is magnetically polarized in-plane with respectto the substrate: and a damping factor of the magnetic free layer isincreased by a factor less than about 2 by contact with the spin Halleffect base layer.
 26. The method of claim 24 wherein: the magnetic freelayer is magnetically polarized perpendicular with respect to thesubstrate; and a contribution to the perpendicular magnetic anisotropyof the magnetic free layer is suitable to achieve a total anisotropyenergy of the magnetic free layer between 40 k_(B)T and 300 k_(B)T. 27.The method of claim 24 wherein the non-magnetic conductor materialcomprises at least one of Pt, Pd, Nb, Mo, Ru, Re, Os, Ir, Au, Tl, Pb andBi conductor materials, and alloys of the foregoing conductor materials.28. The method of claim 24 further comprising measuring at least one ofa sense current and a sense voltage through the magnetoresistive stack.29. The method of claim 24 wherein the spin Hall effect base layercomprises at least one of Pt, Pd, Nb, Mo, Ru, Re, Os, Ir, Au, Tl, Pb andBi conductor materials, and alloys of the foregoing conductor materials.30. The method of claim 24 wherein the spin Hall effect base layercomprises at least one of a tantalum material and a tungsten material.31. The method of claim 24 wherein the spin Hall effect base layercomprises at least in part at least one of a beta phase tungstenmaterial or a beta phase tantalum material.